I read through the thread, but apologize if this was previously mentioned. I have Brio2s and get 10-12 days from size 13 batts. @ ~12 hrs/day. I chose the Brio 2s partly because of the extended batt time of the 13s. The 312s were getting 6-7 days at that point (per the audio tech).
No biggie, but additional days - fewer changes. He also explained that rechargeable were somewhat convenient, but as the battery is recharged over time, there might be degradation in the duration they last during the day. What if they die after 12 hours and you are just getting ready to watch Monday Night Football? Also, if they fail and need replacement = $$$. He steered me away from them at that time. Rechargeble isn’t the reason for my post here.
At the time the Brio 2s were introduced, about 5 yrs ago, I had a several discussions with the Costco tech about the merits of the longer lasting batts and / or rechargeable. He made a comment a couple of times that Bluetooth tech wasn’t embedded in hearing aids because of the additional drain on batteries. I suspect you are losing 1-2 days use on the 312s in the KS9s because of the Bluetooth. Given the cost of batts, not a bad trade-off for the convenience and clarity of the audio.
As an aside, I have been a happy user of an app called Tunity (I have no personal reason or any gain for sharing this - only happy experience). It is a slick app that lets you sit in a crowded/noisy bar or at a family gathering and point your phone to a TV. It connects by Bluetooth, identifies the channel and pipes the audio right into your aids. Nobody even knows tou can hear it. Pretty cool.